Microwave technology brings great advantages for heating of different kinds. For that reason it is used to an increasing extent. Microwaves are considered to cause danger for damages to many organisms including humans and other warm-blooded animals. A fundamental condition at all uses of microwave technology is a reliable shielding against leakage of microwaves.
When microwaves are used in closed chambers such as ovens, etc. shielding is no large problem. Metals are good reflectors for microwaves and do not let through radiation. Even where a certain degree of ventilation is required shielding can, with relative ease, be arranged by perforated plates or nets of a suitable metal. The apertures in the plate or net must be dimensioned not to let radiation through.
The situation becomes more complicated, when microwaves are used for treatment of water damages at ceilings, walls and floors. Examples of such use can be found in the Swedish patent No. 9400751-0, PCT/SE95/00219 and PCT/FI91/00330 where small, mobile, microwave units, consisting of a, downwards open, parallel-epipedic, plate container comprising one or more microwave generators with micro wave outputs in the range 500 to 1500 kw, are used. Suitable dimensions for such units, containing one magnetron with an input effect between 1 and 3 kilowatts, are a quadratic or rectangular opening with sides between 40 and 60 centimetres.
Problematic areas with respect to shielding at such use are the area close to the apparatus, where radiation may leak out at the gap between the apparatus enclosure and the floor or the other surfaces dried, and the rooms situated behind the floor, the wall or the ceiling dried. The first problem has got a simple and good solution, which is described in the Swedish patent No. 9503308-0 and the corresponding international application No PCT/SE96/01170, where widening of the bottom border of the apparatus enclosure is used for obtaining a barrier for microwave radiation. For the second and more difficult problem, penetrating radiation, a solution with good function has been lacking. Instead one has been forced to introduce restrictions with respect to accessibility and use of these rooms. This has made use of microwaves for drying of water damaged building parts more complicated and expensive.
As told, inter alia, in PCT/SE95/00219 use of microwave technique for drying of water damaged building parts involves very large time gains and much less inconvenience for people living in or in other ways using the buildings sanified. Risk of dangerous microwave radiation reduces these advantages, as it may enforce evacuation and closing of rooms that otherwise would have been available during the sanifying phase. Thus it is important to find technique that limits the need for evacuation to the room where the equipment is working only.
PCT/DE96/02231 describes use of microwaves for drying of buildings and building parts. Even if intrusive humidity is mentioned in passing, the description makes it clear that the purpose is elimination of so-called construction humidity, i.e. water in mortar, concrete etc. The shielding aspect is touched upon very superficially. One uses either a metal net placed under the grouting on the outer sides of ceilings, walls and floors, in this way literally converting the room to a xe2x80x9cmicrowave ovenxe2x80x9d, or a mobile reflector plate at the opposite side of the surface being dried. In both cases the condition for obtaining a shielding effect is the accessibility existing during the building phase, but usually not after that the building has been finished and taken into use.
DE 4420649 A1 and DE 9413736 U1 describes use of microwaves for drying and control of fungus attack. With one exception the embodiments concern wooden structures of different kinds. For the drying of a not filled, water damaged wooden floor structures with wood coverings at upper and under sides, covering of both sides with metal foil is proposed. A hole is made in the upper covering, where the microwave source is placed, so that the radiation can penetrate down into the floor structure.
The arrangement presupposes an open floor structure with channels, to make evacuation of humidity possible. Wooden structures of this kind have very low quenching effect on microwaves. This is especially true in dry condition. Thus the covering must reach so far outside the microwave source that the radiation""s decay in air is sufficient for decreasing the intensity to a safe level.
The arrangement cannot be used for concrete structures, filled structures, building bricks etc. where channels for evacuation of humidity are lacking. In such cases a fundamental condition is that at least one side is left free to let out evaporated water. The method is also inconsistent with the requirement for mobility of the microwave source that is a fundamental prerequisite for use of the methods described in PCT/SE95/00219.
None of these earlier publications give any guidance for solving shielding problems arising when microwaves are used for the sanifying of already occupied flats. Such a solution must be safe and cheap. Otherwise, the profitability of microwave methods is decreased in comparison to other alternatives. Further, the shielding must be easy to install and just as easy to remove after finished work, without leaving remaining traces on wall paper and painting. Another desideratum is that the shielding should allow undisturbed activity in rooms next to the space, where sanifying and drying goes on.
The inventor has observed that many building materials have large capacity for quenching microwave radiation with frequency around 2,45 GHz, which is usually used for microwave ovens and similar applications. This is a fundamental prerequisite for heating by microwaves. The quenching corresponds to the desired heating. The quenching depends to a substantial part on the water content in the material. Thus, it decreases with diminishing water content. However, some building materials, which in dry condition contain substantial amounts of crystal water, such as concrete, inclusive light concrete and gypsum, have even after drying sufficient quenching capacity for obtaining reliable protection.
A prerequisite for this is that ceilings, walls or floors are sufficiently thick to cause the quenching required, so that humans and animals without risk for injuries can stay in adjacent spaces. The invention concerns simple means to extend the distance through the quenching material that the radiation has to pass before reaching the air, where the decay is slow.
Thus, this method improves the use of the inherent quenching capacity of the building material by governing the ray path. Besides reliable shielding, the method gives energy gains by using, for evaporation of water, the energy in the radiation that otherwise would have escaped through the floor, wall or ceiling. The method implies that the radiation that penetrates the wall, the floor or the other constructions treated is reflected back into the building material by a metal foil, which has been applied close to the outer surface.
The method of the invention is meant for use at microwave treatment of closed spaces, i.e. spaces limited by walls, floors and ceilings, where it is desirable that humans and/or animals, without risk for injury, can live just outside, under or over the space, where one or more microwave units are working. Use of the method considerably improves the usability of mobile, automatic governed microwave units, such as those described in the mentioned PCT/SE95/00219.
The present invention concerns a method for shielding against penetrating radiation at drying or other treatment of surfaces.
The method of the invention means that a thin, metal foil, especially an aluminium foil is used for partial covering of walls, ceilings, floors and similar surfaces. For surfaces that do not need drying, the covering may be applied to the side, where the microwave unit is placed. For surfaces that have to be dried, the covering is applied to the opposite side of the wall respectively floor. For building materials that in dry condition also contain substantial amounts of water as crystal water and comparatively little voids such as concrete, it is usually sufficient if the outer border of the foil is situated at least 50 cm from the centre of the microwave source. For other building materials, such as wood and brick and constructions, where the wall or the floor structure has air-filled cavities, the distance may have to be increased. As a rule 140 cm are sufficient in most cases. It is no disadvantage, but no advantage either, if the shielding reaches farther out.